The production of visible light (λ=400–700 nm) by means of second- harmonic generation in non-linear optical crystals is a known process. A preferred approach is to use a non-linear material which has been periodically poled. In this technique, the inherent wavelength conversion efficiency of the non-linear crystal is enhanced by imposing a periodic reversal in the orientation of the polarization of the crystal along the direction of light propagation. Potassium Titanyl Phosphate (KTP) is one non-linear crystalline material which is used in a number of applications in non-linear optics, including second- harmonic generation. For example, periodically poled potassium titanyl phosphate (PPKTP) has been used in the frequency doubling of near-infrared laser light to produce visible blue light. See, for example, WIPO Application No. 98/36109 for a detailed description of a method for transforming a crystal of KTP into (PPKTP) in order to permit quasi phase matching, which enhances conversion efficiency. Several workers with KTP have reported that, when used in frequency doubling, or other optical processes where UV, IR and/or visible light is transmitted for extended periods of time through a PPKTP crystal, whether grown by the older hydrothermal technique or by the currently preferred flux method, the crystal is subject to damage in service. Apparently this damage is caused by either or both of the input light (generally IR) and the output light (generally in the visible range or UV range) which results from the frequency doubling.
When used for frequency doubling, a variety of specific problems with the PPKTP crystal have been encountered, including photorefractive damage as manifested by:                i) changes in the size, shape and/or waist position of the frequency doubled beam and also beam astigmatism, and        ii) a decline in the frequency doubled (second harmonic) beam spatial mode quality.        iii) A change in the temperature at which second harmonic generation is optimal. This temperature is known as the phase matching temperature.        
I will hereafter refer to these three effects as BSE for Beam Shift Effect.
Another problem is photochromic damage, one visible aspect of which is called “gray tracking”, which term is used to describe the appearance of discolored regions in the crystal. Gray tracking may be a visible indicia of increased absorption, which effect can significantly reduce the crystal's conversion efficiency and hence the laser's power output. Although the cause of photochromic damage is almost certainly not limited to the effect of the blue or other visible wavelength output light, we refer to this phenomenon as BIA for Blue Induced Absorption. This effect may also be due at least in part to the infrared pump beam which is being frequency doubled to produce the visible output light. If we consider the case of using a 976 nm IR pump laser to generate frequency doubled 488 nm blue light, the consequence of BIA is that the output power of the 976 nm gain chip has to be continuously increased over the operating life of the laser to compensate for the increased absorption (and hence reduced output) of the blue light by the PPKTP crystal. Compensation can be achieved by increasing the pump current to the gain chip, however, there is an upper limit beyond which the gain chip current cannot be safely raised without risking sudden chip failure.
It is by no means clear whether both BIA and BSE result from the same change or changes occurring in the crystal itself, but both effects are believed to occur as a result of the passage through the crystal of the pump and/or frequency doubled radiation for a prolonged period.
Over the past twenty years, a number of prior artworkers have endeavored to understand and/or solve the performance problems associated with the use of KTP and especially PPKTP for frequency conversion and other optical processes. The approaches have involved varying the crystal formation conditions, and/or treatment of the KTP crystal. To date, none of these approaches have proved wholly successful.
It seems clear that part of the problem prior artworkers have encountered in solving the KTP and PPKTP crystal degradation problem has been disagreement as to the mechanism, or more likely mechanisms, involved in such degradation. See, for example, “Nuclear Instruments and Methods in Physics Research” B, 141, pp 472–476 (1998); and J. Appl. Physics 87, 12, pp 8682–8687 (2000). At least some of the prior artworkers have postulated that the damage susceptibility of the PPKTP is due to deviation from stoichiometry (i.e., Potassium ion vacancies) in the crystal lattice. Hence, early workers tried annealing the crystal at the very high temperatures at which there would presumably be some mobility on the part of atoms present in the crystal lattice, in an effort to achieve a more uniform stoichiometry throughout the crystal. Other workers have investigated the effect of potassium non-stoichiometry on the crystal Curie temperature after high temperature (970° C.) heating in air, Appl. Phys. Lett. 67 (13) pp 1941–1943 (1995). Heating a KTP crystal in dry oxygen at 800° C., prior to poling to form PPKTP, has been reported to increase absorption at a wavelength of 500 nm., J. Appl. Phys. 73 (7), 2705 (1992), but conversely, heating in a wet Oxygen atmosphere at 800° C. is said to provide an improved crystal. Still other workers have suggested that synthesis of KTP in an Oxygen atmosphere affords a crystal having a stronger second-harmonic generation (SHG) signal, Solid State Comm. 91, 9, pp 757–759 (1994). However, later workers reported an improvement in transmission in the range 400–550 nm by growing the KTP crystals in an oxygen deficient ambient atmosphere, Appl. Phys. Lett. 69,(8) pp 1032–1034 (1996). The following review article describes much of the currently published literature on KTP and PPKTP: M. N. Satyanarayan, A. N. Deepthy and H. L. Bhat “Potassium Titanyl Phosphate and Its Isomorphs: Growth, Properties, and Applications”, Critical Reviews in Solid State and Materials Sciences, 24, 2, (1999), pp 103–189.
The extensive studies of KTP and/or PPKTP, only a few of which have been referred to above, while doubtless of scientific interest, have not provided a viable procedure for providing a KTP crystal and, in particular, a PPKTP crystal, which has a significantly reduced tendency to develop one or more of the previously enumerated problems, e.g., gray tracking, astigmatism, and other beam quality degradation when subjected to optical radiation for an extended period of service.